R-Type is drenched in gaming memories for me. I first discovered it in the smoky back room of the local arcade, and right from the off I knew it was something special. It was the first time I'd played a shmup where the environment felt such an integral part of the experience, as opposed to something to just crash into. Of course the Force orb was an infamous innovation, and the variety of weaponry lent a strategic feel to the proceedings (though to be fair you can adapt any of the weapons to the immediate task). I plunged many 10 pence pieces into that machine, and it gave me a good grounding for the home versions to follow.
No home version impressed quite like the PC Engine, though. I bought a PC Engine around 1988, and with it I got a couple of nondescript platform games... and R-Type. The platformers were quickly forgotten about and I settled down to a long, long period of playing nothing but R-Type (partly because I couldn't actually afford any more PC Engine games). Every single attack pattern is etched on my brain, and I played that PCE version so much that it became a kind of Zen exercise... true zone gaming.
The problem with R-Type is that it tails off in quality and innovation past level four, and the second half of the game doesn't live up to the promise of the first. Which is why R-Type I for the PC Engine makes an entry into my 100, and not the complete arcade version. R-Type I contains only the first four levels of the game, with the latter half appearing on a separate card, naturally titled R-Type II.